In high frequency (HF) surgery, electrical energy is fed to the tissue to be treated. In this regard, a distinction is generally drawn between monopolar and bipolar application of the high frequency current (HF current).
In a monopolar application, usually only one active electrode is provided, to which the high frequency alternating voltage is applied. The active electrode is situated, for example, at an electrosurgical instrument for cutting and/or coagulating tissue. The application of a neutral electrode to the body of the patient is also required to complete the current circuit through the tissue situated between the active electrode and the neutral electrode. The form of the active electrode depends on the use to which it is put. The surface of the active electrode, by which the alternating current is conducted into the tissue, is relatively small, so that a high current density and consequently a high level of heat generation arise in the direct vicinity of the active electrode.
The current density falls off rapidly with increasing distance away from the active electrode, provided that high current densities do not occur in other body parts as a result of substantial differences in tissue conductivity. The alternating voltage applied to the active electrode is conducted away via the neutral electrode. It should be noted that the neutral electrode is applied over a large area on the body of the patient and presents only a small contact resistance to the high frequency alternating current.
In a bipolar application, two active electrodes are provided, between which the tissue to be treated is accommodated. The flow of current is conducted via the tissue lying between the two active electrodes so that this tissue is heated upon application of an HF current. The majority of the current flows between the two active electrodes.
Sometimes, the neutral electrode is not correctly applied to the patient or the electrode becomes partially detached during treatment. In these cases, the current flow is restricted to the parts of the neutral electrode still making contact, which can lead to a significantly greater impedance at said parts and, in general, to a greater current density within the adjacent tissue. As discussed below with reference to prior art documents, monitoring systems that make an assessment of the application quality of the neutral electrode are known.
For example, DE 10 2004 025 613 B4 discloses a method for determining the contact impedance between two partial electrodes, or electrode sections of a divided neutral electrode, used in high frequency surgery. Herein, the contact impedance is determined between the two electrode sections by an oscillator circuit. It can be assumed that, with the neutral electrode having a large contact area, the contact impedance between the individual sections is significantly lower.
In recent years, treatment methods have been developed whereby relatively large HF currents are applied for a relatively long period. The risk of burning the tissue at the neutral electrode, however, is increased with this method. Thus, even with a correctly applied neutral electrode, damage can still be caused to the tissue depending on the treatment method or the course of the treatment. Theoretically, it is also conceivable to increase the contact area of the neutral electrode, although this is often not practical.
It is therefore necessary to monitor the temperature at the neutral electrode. U.S. application publication no. 2006/0079872 A1 discloses a device for this purpose. According to this publication, a resistor is coupled into the treatment current and the heating of the resistor can be monitored with a heat sensor. The resistor should be selected to substantially simulate the real impedance conditions between the neutral electrode and the active electrode. Suitable selection of the resistor, however, is very difficult because the impedance values change on every application depending on the methods used, the instrument used, the positioning of the instrument and the neutral electrode, the organ being treated, etc.
Other approaches have considered providing commercially available temperature sensors directly on the electrodes. However, the provision of said measurement devices at the electrodes is very complex. In addition, local heating often arise, which may not be detectable by the sensors.
As a rule, the impedance between the two halves of a divided neutral electrode is measured as described above. This measurement provides a guide value for the area of contact, since the resistance is proportional thereto. Furthermore, the current is measured by the neutral electrode and, taken together with the contact resistance, a theoretical power loss incurred at the electrodes is estimated. This power loss can be compared with empirically determined limit values to draw conclusions regarding the temperature at the neutral electrode. However, these approaches are highly error-prone and cannot provide reliable protection against injuring the patient. No account is taken of different tissue types therein.